1. Field of the Invention
The present invention relates to testing of orthogonal frequency division multiplexing (OFDM) transmitters, and in particular, to testing of OFDM transmitters using a vector signal analyzer (VSA).
2. Related Art
As is well known, multiple input, multiple output (MIMO) communication systems use multiple transmitters and receivers to enhance the reliability and signal capacity of the communication link. Testing of each transmitter is usually done by connecting each individual transmitter to a VSA, and repeating measurements for each transmitter sequentially. Alternatively, another method involves connecting each transmitter to its own VSA and performing the tests simultaneously. Accordingly, the first method only requires one VSA, but significantly more time, while the second method requires multiple VSA systems, but significantly less time.
Traditionally testing of wireless devices has involved testing one active transmitter at a time. Even if the device has offered multiple transmitters, they have typically not been operated in parallel. However, people are constantly trying to increase the data rate. In the past this has been achieved by using more complex modulation and higher bandwidth. These methods have used a single transmitter so measurements could be performed with a single input test instrument.
With the introduction of MIMO technology, multiple parallel transmitters are used to increase the allowable data rate in a given bandwidth by having the individual transmitters carry separate information using the same frequency and bandwidth for transmission. During normal operation, system requires multi-path for reliably transmitting the parallel data streams over the same bandwidth simultaneously. The system relies on advanced signal processing to separate the different transmit signals in the required multiple receivers. The receivers separate and extract the data transmitted by the multiple transmitters. Accordingly, multiple parallel receivers are needed to fully analyze a true MIMO signal, and one can no longer use a single input test instrument to fully analyze the transmitted signal.
This is particularly true for research and development (R&D) testing, where one needs to get as much information as possible about the device under test (DUT). However, for production testing, one may not need as much information, as one is really testing to determine if the DUT is correctly assembled and if all components are fully functional. It is assumed that all major components (e.g., chips) have already been tested, and that the design being produced is verified to work correctly if the assembly is complete and correct, thereby obviating a need for as detailed of a test setup.
From a production perspective, one is looking to have the lowest possible test cost that satisfies full coverage of the required tests. Production testing usually includes both product verification, and often more importantly product calibration. During the product calibration, the performance of the device is adjusted to meet the desired performance.
Optimizing costs of testing in production includes ensuring the fastest possible test time with reasonably priced test equipment. Testing MIMO transmitters would indicate that one could utilize parallel test equipment, such that each transmitter is tested in parallel. This will add little to the test time compared to a traditional device, but will double the cost of the test setup, thus increasing the overall test cost.
Since modem test equipment offers significantly more signal processing capability, options other than simply performing all tests in parallel do exist. As noted, it may not be necessary to measure all parameters of a DUT in production; often one can simply measure the parameters that are expected to change in production devices. This includes identifying failing components and assembly problems, as well as the ability to calibrate the individual transmitters' performance to be close to optimal.